The present invention relates to coating surfaces which may come into contact with liquid metal particularly steel and more particularly the invention relates to the coating of such surfaces with soot or carbon black under utilization of combustion of combustible gas yielding the deposit.
It is known to provide surfaces of workpieces with a coating of soot or carbon black when the particular surface may and is expected to come in contact with liquid metal such as steel. A typical example here is a mold for continuous casting. The soot to be deposited is usually developed by a flame resulting from the combustion of a gas blend containing acetylene while the flame is directed to the surface to be coated. The resulting soot particles will in fact deposit onto that surface.
The specific purpose of such a soot and carbon black layer is to reduce on one hand the wear of the surface of the mold wall or the like while on the other hand the soot layer facilitates separation of the casting ingot from the mold and its walls and surfaces. In addition, the soot particles prevent penetration (or are supposed to prevent) of liquid metal into the mold particularly into any cracks or fissures, separation gaps and so forth, as may be in the part.
The soot and carbon black layers made in accordance with the known method are only of limited use since it was found that no matter how carefully they are worked there is a certain nonuniformity in thickness. Unfortunately also, a soot layer does not adhere very well for any solid surface and may readily separate from such a surface on which it has been deposited. Here it has been attempted to improve adhesion of the soot layer to the substrate by adding some kind of drive or propellant gas to the organic gas blend that is combusted. The driving or propelling gas in effect causes the gas molecules and particles as they are developed in the flame, to be directed towards the surface to be coated at an increased speed. This way the soot layer is deposited in a more forceful depositing process and it was found that the quality as far as adhesion is concerned is indeed improved. However, that is only one advantage; the other drawbacks remain.
It was found otherwise that developments in the field of continuous casting, particularly casting of strip or other flat ingots made of steel and if made conventionally while being provided with a soot layer at a strip thickness from half to 40 mm are simply not adequately provided for in that fashion. On casting steel strip in the stated thickness range a so called cooling strip is used upon which the steel is deposited. The cooling strip is made of steel and has the function of its designation. In order to avoid welding of the liquid steel with the cooling strip which is cooled from the underside, it is necessary to provide that cooling strip with coating that prevents such a welding. It is believed that presently no known method exists by means of which a durable layer can be provided under utilization of hardening, nitrating, oxidizing or under utilization of spray depositing, weld depositing, bonding, adhering etc.